Nest Thermostat blinking a red light

A blinking red light near the top of your thermostat’s display means that the battery charge is very low but it’s being charged. There are different troubleshooting steps depending on when you see the red light. Let's get this sorted out by trying these steps:

1. Your thermostat should automatically turn on.

Your thermostat should automatically turn on when the battery is charged enough. Depending on how low the battery is, you may only have to wait several minutes. But an extremely low battery can take up to an hour to charge.

2. Speed up charging with a USB cable
Pull off the Nest display and use the USB port on the back to charge it with a USB cable and wall charger. Depending on the model you have it will be either mini or micro USB. Note: Do not use a computer USB port or other unpowered USB port for charging.
The the display should eventually light up with a message asking you to connect it to the base.
Reconnect the display to the thermostat base. Make sure you press firmly until you hear it click into place.
3. If the blinking red light doesn’t go away or keeps coming back A persistent blinking red light can mean a couple of things. You’ll need to check the power readings on your thermostat.